Sorry, just got around to marathoning a few games. I got almost 4 hours of straight game time.
Played with the brightest setting, power save off, rumble on. Games I played were Duck Tales, The Wonderful 101 demo, and Earthbound. Overall this makes me pretty satisfied, noticeably longer (obviously) than the stock battery pack, and the weight difference feels almost negligible.
I don't have any concrete numbers to compare mine to, but I remember I could hardly get two or so hours while playing Tekken Tag Tournament 2 on the gamepad (with the same settings). I may do some more tests if I get requests, it's just a little hard since I have to marathon through to get the numbers (idling the controller does not give reliable results, i've tried this already).
Edit: Had another marathon. Went for a solid 6 and a half hours (before I realized I need some rest, will resume tomorrow). Playing on the lowest brightness, power saving on, speakers running at full and no rumble. Surprised how long this has gone.